A 59-foot section of the upper Cobourg formation (upper Middle Ordovician) at Colborne, Ontario, was samples at 2-foot intervals without regard to bedding. The limestone is thin-bedded biomicrite and biosparite,

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with the latter occurring consistently only in the lower two-thirds of the section. Although the -40 +140-mesh insolubles increase slightly toward the top, the allogenic "heavies" are more abundant in the top and bottom third than in the middle. Some microfossils such as brachiopods, gastropods, and sponge spicules have different abundances in the lower and upper part of the section. Approximately 3,500 conodonts, assigned to 27 genera, vary in concentration from less than 2 to more than 200 individuals per 100 grams, with the average about 40. However, exceptionally low concentrations are adjacent to exceptionally high. The moving averages of both concentrations and numbers of genera show a decreasing trend toward the top of the section. The different conodont elements--that is, cone , compound, and platform types--show a stratigraphic variation per sample for concentration in the rock and percentage of total fauna, suggesting that some types were carried by different animals. Stratigraphically, the distribution of certain genera seem to show a relation to the occurrence of the biosparite, the heavy minerals, and other microfossils. The trends and variations in the Colborne quarry are probably local, but the relation between petrographic and paleontologic factors suggests that environmental conditions probably controlled the conodont distribution.

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